1. Field of Invention
This invention relates to a holding device and a method of coating hollow cylindrical objects using the device. More specifically, the present invention provides a holding device and a method of selectively and efficiently coating one or more hollow cylindrical objects, such as stents and catheters, while preventing the stent's interior surfaces from receiving coating material.
2. Background of the Invention
Coatings are often applied to medical implants, such as pacemakers, vascular grafts, catheters, stents, heart valves, tissues or sensors to have desired effects and increase their effectiveness. These coatings may deliver a therapeutic agent to the lumen that reduces smooth muscle tissue proliferation or restenosis and may comprise a polymer carrier. Furthermore, implants may be coated to improve surface properties such as lubriciousness, to achieve enhanced biocompatibility and to control the timing and rate of release of the therapeutic agent being delivered. Balloon delivery systems, stent grafts and expandable stents are specific examples of implants that may be coated and inserted within the body. Stents such as described in U.S. Pat. No. 4,733,665 are tiny, expandable mesh tubes supporting the inner walls of a lumen used to restore adequate blood flow to the heart and other organs.
Conventionally, coatings are applied to the inner and outer surface of a stent in a number of ways, including, though not limited to, dip coating, dispensing or spray coating.
Applying a drug-containing coating uniformly on the inner and outer surface of the medical device can however lead to adverse drug effects or delivery to non-target tissue due to exposure of the coating material to the bloodstream. Additionally, it is desirable to coat only the outer surface of the stent to avoid excessive use of expensive coating material.
It is known to mask a device by placing a temporary sleeve over a portion of the medical device or by using a special fixture comprising masking means contacting the inner surface of the device to prevent the coating from coming in contact with the inner portion of the device. A drawback of such masking means is the high degree of surface contact between the stent and the masking means that may cause sticking of the masking means to the stent.
It is also known to use special fixtures having a polygonal shape that extend through the inner hollow section of the stent to cover its inner surface as depicted in FIG. 1. When using these fixtures there is a risk of coating defects, as illustrated in FIG. 1. Droplets 41 passing through the openings of the stent 1 may deposit on the fixture or on the inner surface of the stent 1. The excess coating material forms a film 43 on the surface of the holder that leads to coating material accumulation 44 on the contact points between fixture 6 and stent 1 resulting in inhomogeneous coatings and coating defects.
When the coated stent is removed from the fixture the stent may stick to the masking means and excess coating may remain on and/or between the struts or some of the coating may be removed from the stent leaving bare areas. Inhomogeneous coatings and uncoated areas on the stent surface may compromise the implant's effectiveness due to potential complications arising from an inhomogeneous distribution of the therapeutic agent at the target site.
Thus, conventional stent holding devices have several drawbacks that may result in increased manufacturing costs of stents and in coating defects as described above leading to time consuming inspection and product scrap. A repeatable process of selectively coating the outer surface of a stent may therefore not be ensured. Finally, stent holding devices known by the prior art are not designed to support and rotate multiple stents simultaneously to efficiently apply a coating to the stents.
Accordingly, a shortcoming of the conventional coating techniques is the inability to coat selectively and repeatably the outer surface of the stent, while preventing coating defects. Thus, there is a need for a system and method for efficiently applying a high quality coating only on the exterior surface of a stent, while preventing coating application on the interior surface and coating defects.